1. Field of the Invention
The present invention relates to a brake control device for controlling a braking force applied to wheels to prevent locking of the wheels when a brake is put on to a vehicle such as an automobile mounted with tires, a brake control method therefor, and a computer readable recording medium having a program for the method recorded therein. In particular, the present invention relates to a brake control device with which a time lag before the brake control starts is shortened to reduce a braking distance of a vehicle when the brake is put on to the vehicle, a brake control method therefor, and a recording medium.
2. Description of the Related Art
Conventionally, various techniques for a control method of an anti-lock brake system (hereinafter referred to as ABS) have been proposed.
For example, in one technique, a wheel velocity of a wheel is detected every moment, and this wheel velocity is processed by time differentiation in a computing unit mounted on a vehicle to find a wheel deceleration. Then, if the wheel deceleration thus obtained comes across less than a predetermined value set in advance, a brake pressure to be applied get decreased to prevent locking of the tire of the wheel (prior art 1).
On the other hand, JP 10-71943 A discloses the ABS described below (prior art 2).
FIG. 5 is a block diagram showing a control start discriminating device in the ABS disclosed by the prior art 2. FIG. 6 is a graph showing dependencies of a braking torque and a braking torque gradient on a slip velocity, in which the slip velocity is assigned to the horizontal axis and the braking torque is assigned to the vertical axis. FIG. 7 is a flow chart showing a control start discrimination of the ABS by the conventional control start discriminating device.
As shown in FIG. 5, a control start discriminating device 100 includes a wheel velocity detecting unit 101 for detecting a wheel velocity at each predetermined sampling time, a torque gradient estimating unit 102 for estimating a gradient of a braking torque with respect to a slip velocity from time-series data of the detected wheel velocity, and a judging unit 103 for comparing the estimated braking torque gradient with a reference value to judge a control starting point and a control finishing point of a control unit 104 of the ABS. Then, the control starting point of the ABS is judged based on the gradient of the braking torque.
As shown in FIG. 6, a curve D is a curve indicating a change in a braking torque with respect to a slip velocity. On this curve D, an inclination of a tangent to the curve D at an arbitrary slip velocity stands for a braking torque gradient. Plus or minus of this control torque gradient changes at a peak xcexc point where the braking torque gets maximized. The braking torque gradient becomes zero at the peak xcexc point, becomes positive in an area where a slip velocity is smaller than that at the peak xcexc point, and becomes negative in an area where a slip velocity is larger than that at the peak xcexc point.
Control start discrimination of the ABS in the above-mentioned prior art 2 will be described. As shown in FIG. 7, first, the control start discriminating device 100 judges whether a braking torque gradient estimated by the torque gradient estimating unit 102 is larger or smaller than a reference value (step S100). If it is judged that the braking torque gradient is larger than the reference value in the step S100, the control start discriminating device 100 sets the ABS in a non-anti-lock braking mode (step S102). On the other hand, if it is judged that the braking torque gradient is smaller than the reference value, the control start discriminating device 100 sets the ABS in an anti-lock braking mode (step S101). Next, after setting the ABS in the anti-lock braking mode, the control start discriminating device 100 controls a brake pressure to reduce a braking force, thereby preventing locking of tires (step S103).
In this way, in the prior art 2, the control start discriminating device 100 judges a control starting point by checking a braking torque gradient. This judgment makes use of the fact that a slip velocity reaches the peak xcexc point when the braking torque gradient is zero.
However, in the prior art 1, a braking force is required to be applied until a desired slip ratio is reached or a desired slip velocity is exceeded once in order to grasp an optimum slip ratio or an optimum slip velocity corresponding to an initial state of braking, that is, a slip ratio or a slip velocity at which a braking torque becomes maximum. Since the optimum slip ratio or the optimum slip velocity is calculated, extra time is required and a time lag before the control starts is lengthened. Thus, there is a problem in that it takes time to bring tires into a state of the desired slip ratio or slip velocity and a braking distance gets long. In particular, it is extremely important to reduce a braking distance on slippery road surfaces such as a wet road surface and a frozen road surface.
On the other hand, in the prior art 2, a braking torque gradient is used for judging a control starting point. Since this braking torque gradient is calculated in advance using a history of wheel velocities, a calculation time is required. Therefore, a time lag before the control starts is lengthened. In addition, since the control of a braking force is simply decided according to plus or minus of the braking torque gradient qualitatively in the anti-lock braking mode, it is not guaranteed that a braking torque is always maximized when the brake is put on.
The present invention has been devised to solve such problems of the prior art, and therefore has an object to provide a brake control device that, when the brakes are put on to a vehicle, can control a braking force quantitatively such that a slip ratio becomes optimum and can reduce a time lag before the control starts to reduce a braking distance of the vehicle, a brake control method therefor, and a computer readable recording medium having a program for the brake control method recorded therein.
The invention provides a brake control device of a vehicle which applies a braking force to a wheel mounted with a tire to brake the vehicle, comprising: a memory device storing therein a friction characteristic of the tire with respect to a vehicle velocity of the vehicle or a wheel velocity of the wheel; a detecting unit for detecting the vehicle velocity of the vehicle and the wheel velocity of the wheel; a signal generating unit for, when the braking force is applied to the wheel, finding a slip ratio based on the vehicle velocity and the wheel velocity detected by the detecting unit and, finding an optimum slip ratio, at which a maximum friction force is generated by the tire at the vehicle velocity or the wheel velocity detected by the detecting unit, by using the characteristic called from the memory device and, if a difference found by subtracting the optimum slip ratio from the found slip ratio is equal to or more than a predetermined value, generating a control signal for controlling the braking force according to the difference; and a braking unit for controlling the braking force based on the generated control signal to adjust the wheel velocity.
It is preferable that the characteristic is supplied to the memory device from an external device or a data recording medium, and it is also preferable that the characteristic includes a rate of change of the optimum slip ratio with respect to the vehicle velocity or the wheel velocity, and the rate of change has a negative value. And it is still also preferable that the memory device stores the characteristic that the optimum slip ratio changes linearly with respect to the vehicle velocity or the wheel velocity.
For example, when the vehicle velocity or the wheel velocity is assumed to be a velocity V (km/h), the optimum slip ratio with respect to the velocity V is assumed to be Sd (%), the memory device may preferably store coefficients xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d represented by a following expression (1) for the characteristic:
Sd=axc3x97V+bxe2x80x83xe2x80x83(1)
Then, the coefficient xe2x80x9caxe2x80x9d is preferably a value in a range of xe2x88x920.2 to xe2x88x920.01.
The invention also provides a brake control device of a vehicle which applies a braking force to a wheel mounted with a tire to brake the vehicle, comprising: a memory device storing therein a friction characteristic of the tire with respect to a vehicle velocity of the vehicle or a wheel velocity of the wheel; a detecting unit for detecting the vehicle velocity of the vehicle and the wheel velocity of the wheel; a signal generating unit for, when the braking force is applied to the wheel, finding a slip velocity based on the vehicle velocity and the wheel velocity detected by the detecting unit and, finding an optimum slip velocity, at which a maximum friction force is generated by the tire at the vehicle velocity or the wheel velocity detected by the detecting device, by using the characteristic called from the memory device and, if a difference found by subtracting the optimum slip velocity from the found slip velocity is equal to or more than a predetermined value, generating a control signal for controlling the braking force according to the difference; and a braking unit for controlling the braking force based on the generated control signal to adjust the wheel velocity.
It is preferable that the characteristic is supplied to the memory device from an external device or a data recording medium, and it is also preferable that the characteristic includes a rate of change of the optimum slip velocity with respect to the vehicle velocity or the wheel velocity, and the rate of change has a negative value. And it is still also preferable that the memory device stores the characteristic that the optimum slip velocity changes quadratically with respect to the vehicle velocity or the wheel velocity.
For example, when the vehicle velocity or the wheel velocity is assumed to be a velocity V (km/h), and the optimum slip velocity is assumed to be Sv (km/h), the memory device may preferably store coefficients xe2x80x9caxe2x80x9d and xe2x80x9cbxe2x80x9d represented by a following expression (2) for the characteristic:
Sv=(axc3x97V+b)xc3x97V/100xe2x80x83xe2x80x83(2)
Then, the coefficient xe2x80x9caxe2x80x9d is preferably a value in a range of xe2x88x920.2 to xe2x88x920.01.
The invention provides a brake control method for a vehicle which applies a braking force to a wheel mounted with a tire to brake the vehicle, comprising: detecting a vehicle velocity of the vehicle and a wheel velocity of the wheel; finding a slip ratio based on the detected vehicle velocity and wheel velocity; when the braking force is applied to the wheel, finding an optimum slip ratio at which a maximum friction force is generated by the tire at the detected vehicle velocity or wheel velocity, by using a friction characteristic of the tire with respect to the vehicle velocity or the wheel velocity; if a difference found by subtracting the optimum slip ratio from the found slip ratio is equal to or more than a predetermined value, generating a control signal for controlling the braking force according to the difference; and controlling the braking force based on the generated control signal to adjust the wheel velocity.
The invention also provides a brake control method for a vehicle which applies a braking force to a wheel mounted with a tire to brake the vehicle, comprising: detecting a vehicle velocity of the vehicle and a wheel velocity of the wheel; finding a slip velocity based on the detected vehicle velocity and wheel velocity; when the braking force is applied to the wheel, finding an optimum slip velocity, at which a maximum friction force is generated by the tire at the detected vehicle velocity or wheel velocity, by using a friction characteristic of the tire with respect to the vehicle velocity or the wheel velocity; if a difference found by subtracting the optimum slip velocity from the found slip velocity is equal to or more than a predetermined value, generating a control signal for controlling the braking force according to the difference; and controlling the braking force based on the generated control signal to adjust the wheel velocity.
The invention provides a recording medium having recorded therein a computer readable program for causing a computer to perform adjustment of a braking force, when the braking force is applied to a wheel mounted with a tire to brake a vehicle, wherein the program includes: a procedure for causing a computing unit of the computer to calculate a slip ratio based on a vehicle velocity of the vehicle and a wheel velocity of the wheel; a procedure for calling a friction characteristic of the tire with respect to the vehicle velocity or the wheel velocity stored in a memory device of the computer in advance and causing the computing unit to calculate an optimum slip ratio, at which a maximum friction force is generated by the tire at the vehicle velocity or the wheel velocity, by using the called characteristic; a procedure for, if a difference found by subtracting the optimum slip ratio from the calculated slip ratio is equal to or larger than a predetermined value, causing the computing unit to generate a control signal for controlling the braking force according to the difference.
The invention also provides a recording medium having recorded therein a computer readable program for causing a computer to perform adjustment of a braking force, when the braking force is applied to a wheel mounted with a tire to brake a vehicle, wherein the program includes: a procedure for causing a computing unit of the computer to calculate a slip velocity based on a vehicle velocity of the vehicle and a wheel velocity of the wheel; a procedure for calling a friction characteristic of the tires with respect to the vehicle velocity or the wheel velocity stored in a memory device of the computer in advance and causing the computing unit to calculate an optimum slip velocity, at which a maximum friction force is generated by the tire at the vehicle velocity or the wheel velocity, by using the called characteristic; a procedure for, if a difference found by subtracting the optimum slip velocity from the calculated slip velocity is equal to or larger than a predetermined value, causing the computing unit to generate a control signal for controlling the braking force according to the difference.